The present invention relates generally to code division multiple access (CDMA) communications systems. More specifically, but without limitation thereto, the present invention relates to a method for reducing the number of CDMA codes required for a group of subscribers serviced by multiple transponder platforms for synchronous CDMA communications.
Traditionally, when multiple satellites become available over a given geographic location, two or more nearby users may not use the same frequency spectrum or code space due to interference. Also, as the number of subscribers within a service area increases, the frequency bandwidth, the number of CDMA codes, or both must be increased to avoid interference from messages intended for other subscribers. The number of subscribers is therefore limited by the frequency bandwidth and the number of CDMA codes.
Methods for reducing the number of CDMA codes for a service area effectively increase the bandwidth of the frequency spectrum by making a greater portion of the bandwidth available for subscriber communication rather than for separating messages intended for different subscribers.
Although multiple transponder platforms such as satellites increase the system availability, their full potential has been unrealized because of the limit on the number of users imposed by the assigned frequency bandwidth and the number of available codes. In conventional asynchronous CDMA single satellite communication systems, unique CDMA codes are assigned to each user to ensure that information directed to one subscriber does not interfere with information directed to another subscriber. Similarly, in multiple satellite communication systems, when two or more satellites are serving in the same geographical location, unique CDMA codes within the same frequency bandwidth may be used to distinguish each subscriber. A problem with using the same CDMA code for multiple subscribers is that mutual interference prevents the proper decoding of information because the omnidirectional receiving antennas of the subscribers' terminals lack the capability to discriminate spatially among the satellites.